Water is the dominant heat transfer fluid used in industrial heating and cooling due to its relatively high heat capacity and latent heat of evaporation. Water has corrosive properties and can be scale forming so water treatment additives, such as corrosion inhibitors, scale inhibitors and micro biocides are typically used in heat transfer fluids. Solutions of a suitable organic chemical (most often ethylene glycol, diethylene glycol, or propylene glycol) are used when the water-based coolant has to withstand temperatures below 0° C., or when its boiling point has to be raised. Water is used in evaporative cooling systems, chilled water closed loops, thermal storage systems, solar collectors, spray water for metal casting are a few examples. Kemmer describes in more detail the variety of industrial heat transfer systems. F. N. Kenner, The Nalco Water Handbook, 2nd Ed., McGraw-Hill (1988).
Low thermal conductivity, corrosivity, viscosity, freezing points, boiling points and low heat capacities are the primary limitations in the development of energy-efficient heat transfer fluids required in many industrial applications. To overcome some of these limitations, a new class of heat transfer fluids called nanofluids has been developed by suspending nanocrystalline particles in liquids such as water, oil, or ethylene glycol. Purpose-designed nanoparticles of, e.g., CuO, alumina, titanium dioxide, carbon nanotubes, silica, or metals (e.g., copper, or silver nanorods) dispersed into the carrier liquid the enhances the heat transfer capabilities of the resulting coolant compared to the carrier liquid alone. Wang et al., J. Chem. Eng., vol. 25, no. 4 (2008). The enhancement can be theoretically as high as 350%.
A nanofluid is a fluid containing nanometer-sized particles, called nanoparticles. These fluids are engineered colloidal suspensions of nanoparticles in a base fluid. The nanoparticles used in nanofluids are typically made of metals, oxides, carbides, or carbon nanotubes. Common base fluids include water, ethylene glycol and oil.
Nanofluids have novel properties that make them potentially useful in many applications in heat transfer, including microelectronics, fuel cells, pharmaceutical processes, and hybrid-powered engines as detailed in Das et al.'s, Nanofluids: Science and Technology, Wiley-Interscience, p. 397 (2007), engine cooling/vehicle thermal management, domestic refrigerator, chiller, heat exchanger, nuclear reactors, in grinding, in machining, in space, defense and ships, and in boiler flue gas temperature reduction. They exhibit enhanced thermal conductivity and the convective heat transfer coefficient compared to the base fluid.http://en.wikipedia.org/wiki/Nanofluid - cite note-kakac-3 Kakaç et al., Review of Convective Heat Transfer Enhancement with Nanofluids, Int'l J. of Heat and Mass Transfer, (Elsevier) 52: 3187-3196 (February 2009); Bi et al., Application of Nanoparticles in Domestic Refrigerators, Applied Thermal. Eng'g (2008); and Prasher et al., Thermal Conductivity of Nanoscale Colloidal Solutions (Nanofluids), Physics Review Letters, 94, 025901 (2005).
The term nanoparticles is defined to mean a particle having a major dimension of less than 100 nanometers. For a spherical particle, the major dimension is the diameter of the sphere; for particles that are not spherical, the major dimension is the longest dimension. The resulting nanofluids possess extremely high thermal conductivities compared to the liquids without dispersed nanocrystalline particles. For example, 5 vol % of nanocrystalline copper-oxide particles suspended in water results in an improvement in thermal conductivity of almost 60% compared to water without nanoparticles. Choi et al.'s U.S. Pat. No. 6,221,275 (“Choi”).
Broad industrial use of nanofluids is limited by the cost and availability of materials, and also the stability of the materials. For example the metallic copper nano particles discussed by Choi require vapor deposition of copper into solution. Some nano particles are manufactured by solid state grinding operations which are energy intensive and time consuming. See, e.g., Kuldip et al.'s, U.S. Pat. No. 5,958,282 (“Kuldip”); and Lu et al.'s, U.S. Pat. No. 7,892,520 (“Lu”).
One type of nanofluids are ferrofluids. Ferrofluids are colloidal liquids made of nanoscale ferromagnetic, or ferrimagnetic, particles suspended in a carrier fluid—usually an organic solvent or water. Each tiny particle is thoroughly coated with a surfactant to inhibit clumping. Ferrofluids are composed of nanoscale particles (typically having a diameter of no more than about 10 nm) of magnetite, hematite or some other compound containing iron. This is small enough for thermal agitation to disperse them evenly within a carrier fluid, and for them to contribute to the overall magnetic response of the fluid. The surfactants used to coat the nanoparticles include, but are not limited to oleic acid, tetramethylammonium hydroxide, citric acid, soy lecithin. These surfactants prevent the nanoparticles from clumping together, ensuring that the particles do not form aggregates that become too heavy to be held in suspension by Brownian motion.
A surfactant typically has a polar head and non-polar tail (or vice versa), one of which adsorbs onto a nanoparticle, while the un-adsorbed non-polar tail (or polar head) sticks out into the carrier medium. When a sufficient quantity of the surfactant is present, the surfactant coating on the nanoparticle will form an inverse or regular micelle, respectively, around the particle. Steric repulsion between the similarly encased nanoparticles will then suppress or prevent agglomeration of the particles.
Ferrofluids are commonly used in loudspeakers to remove heat from the voice coil, and to passively damp the movement of the cone as detailed in King's U.S. Pat. No. 4,017,694 (“King”). They reside in what would normally be the air gap around the voice coil, held in place by the speaker's magnet. Because ferrofluids are paramagnetic, they obey Curie's law, and thus become less magnetic at higher temperatures. A strong magnet placed near the voice coil (which produces heat) will attract cold ferrofluid more readily than hot ferrofluid, thereby forcing the heated ferrofluid away from the electric voice coil and toward a heat sink. This is an efficient cooling method which requires no additional energy input.
Ferrofluids containing magnetite can be prepared by combining the appropriate amounts of an Fe(II) salt and an Fe(III) salt in basic solution, a combination that causes the mixed valence oxide, Fe3O4, to precipitate from solution according to reaction [1]. Ellis et al., Teaching General Chemistry: A Materials Science Companion, American Chemical Society (1993); Enzel et al., Preparation and Properties of an Aqueous-Based Ferrofluid, J. Chem. Educ., Vol. 76, no. 7, p. 943 (1999); Cabuil, Magnetic Nanoparticles: Preparation and Properties, Dekker Encyclopedia of NanoScience and NanoTechnology, ch. 119 (2004).2FeCl3+FeCl2+8NH3+4H2O→Fe3O4+8NH4Cl   [1]
Researchers have prepared ferrofluids containing small particles of ferromagnetic metals, such as cobalt and iron, as well as magnetic compounds, such as manganese zinc ferrite, ZnxMn1-xFe2O4. (0<x<1; this is a family of solid solutions). But by far, the most work has been conducted on ferrofluids containing small particles of magnetite, Fe3O4. See, Cabuil.
Despite significant research on nanofluids and ferrofluids, broad industrial application for heat transfer enhancement have remained elusive beyond the domain of electrical devices, small refrigeration units and heat pipes. Expansion of this technology has been limited due to high manufacturing costs, long term stability of the nanofluids, considerations for the environmental impact of nano materials and for the interaction with other water treatment additives.
The following invention discloses a method for cost effective application and control of ferrofluids to a broader class of industrial heat transfer processes than has been previously disclosed. This was accomplished through the preparation of a consistent ferrofluid with particles typically having a size of less than 5 nm diameter and the discovery that this fluid could be added at economical levels to heat transfer fluids and both monitored and controlled to achieve desired improvements in heat transfer efficiency.